1. Field of the Invention
The present invention relates to an object information acquiring apparatus.
2. Description of the Related Art
Studies on optical imaging techniques have been actively carried out which obtain in vivo information by irradiating light from a light source such as laser to a living body to propagate light in vivo and detect the propagated light and the like. One of the optical imaging techniques which has been proposed is photoacoustic tomography (PAT) (M. Xu and L. V. Wang “Photoacoustic imaging in biomedicine”, Review of Scientific Instruments, 77, 041101 (2006)). Photoacoustic tomography is a technique in which pulsed light generated from a light source is irradiated to a living body, acoustic waves generated from body tissues which have absorbed light energy propagated and diffused in vivo are detected at a plurality of points and the signals thereof are analyzed and processed to visualize in vivo information. By this technique, the in vivo distribution of the value of optical characteristics, particularly the distribution of the optical energy absorbed density can be acquired. Such an in vivo imaging technique is also referred to as image reconstruction.
According to M. Xu and L. V. Wang “Photoacoustic imaging in biomedicine”, Review of Scientific Instruments, 77, 041101 (2006), the acoustic pressure (P) of acoustic waves (photoacoustic waves) obtained from an absorber in vivo by light absorption in photoacoustic tomography can be represented by the following formula (1):P=Γ·μa·Φ  (1)wherein Γ is the Gruneisen coefficient which is a characteristic value of elasticity and is obtained by dividing the product of the volume expansion coefficient (β) and the square of the speed of sound (c) by the specific heat (Cp); μa is the absorption coefficient of the absorber and Φ is the light quantity in a local area (the light quantity irradiated to the absorber).
It is known that the Gruneisen coefficient Γ is almost constant when a tissue is identified. Accordingly, by performing at a plurality of points time-resolved measurement of change in the acoustic pressure P, which corresponds to the degree of an acoustic wave, the product of the absorption coefficient μa and the light quantity Φ, i.e., the distribution of the optical energy absorbed density can be obtained.
Japanese Patent Application Laid-open No. 2009-018153 discloses an example of an apparatus based on photoacoustic tomography. This apparatus utilizes relative positional information between an irradiated region and a light absorber in vivo and the acoustic pressure of acoustic waves when signals based on photoacoustic waves are analyzed to determine the distribution of the value of optical characteristics. Accordingly, it is sought to obtain the in vivo distribution of the value of optical characteristics with high resolution, particularly the distribution of the light absorption coefficient or the in vivo average equivalent attenuation coefficient and to accurately visualize them.
Japanese Patent Application Laid-open No. 2010-088627 discloses another example of an apparatus based on photoacoustic tomography. This apparatus utilizes the in vivo distribution of the light quantity determined based on the shape of a living body in order to obtain in vivo information from electric signals based on photoacoustic waves.    Patent Literature 1: Japanese Patent Application Laid-open No. 2009-018153    Patent Literature 2: Japanese Patent Application Laid-open No. 2010-088627    Non Patent Literature 1: M, Xu, L. V. Wang “Photoacoustic imaging in biomedicine”, Review of scientific instruments, 77, 041101 (2006)